Halogen Oxidation and Halogen Photoelimination Chemistry of a Platinum–Rhodium Heterobimetallic Core

The heterobimetallic complexes, PtRh­(tfepma)2(CNtBu)­X3 (X = Cl, Br), are assembled by the treatment of Pt­(cod)­X2 (cod =1,5-cyclooctadiene) with {Rh­(cod)­X}2, in the presence of tert-butylisonitrile (CNtBu) and tfepma (tfepma = bis­(trifluoroethoxyl)­phosphinomethylamine). The neutral complexes contain Pt–Rh single bonds with metal–metal separations of 2.6360(3) and 2.6503(7) Å between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPtIRhIIX2 cores with suitable halide sources (PhICl2 or Br2) furnishes PtRh­(tfepma)2(CNtBu)­X5, which preserves a Pt–Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a meridional geometry, which slowly transforms to a facial arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the mer- or fac-Cl3PtIIIRhIICl2 isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPtIRhIICl2 core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the fac-ClPtIRhIICl2 isomer, details of the X2 photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (4 s–1. As M–X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage.